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A person's mental ability as a child could well be an indicator of their chances of surviving to a ripe old age, according to a landmark study which has followed up on surveys carried out in the first half of the last century.

"This is by far the longest follow-up study of mental ability differences in the scientific literature," said Deary, who led the research. The study was remarkable because it spanned a long time period, included a large number of people and managed to get around some of the problems faced by previous studies into how age affects mental ability.

As the body ages, so does the brain, and as it does, some people lose their mental ability. This loss can become especially pronounced as we reach our seventies and beyond. Because the quality of life in old age is influenced by how well mental ability is maintained, considerable research is now being carried out on how ageing affects our ability to think, reason and remember. Unfortunately, most studies on the ageing of our mental or cognitive ability are limited by not knowing what a person's mental ability was when they were young.

"Here's the problem," said Deary. "Examine the mental abilities of a group of older people and some score better than others on tests of memory, reasoning, perception, and so forth. That does not provide information about what the slings and arrows of a half-century's fortunes have wreaked on the brain. The high scorers might always have scored highly. The low scorers might always have been practical rather than cerebral; commonsensical rather than brainy."

A number of years ago, however, Deary came across a golden opportunity to investigate how age affects mental ability in the form of some country-wide mental aptitude assessments of Scottish children carried out early last century. The assessments, known as the Scottish Mental Survey, were originally designed to help inform educational policy at the time. One assessment of 87,498 eleven-year-olds was carried out in 1932. The other, done in 1947, used the same test on another group of 70,805 children of the same age.

The data provided excellent baseline data for the team of psychologists, biologists, sociologists and public health officials, working with Deary, to study the social, educational, medical, psychological and genetic factors that assist people, over a lifetime, to preserve their mental abilities. Over a thousand of the people who sat for the original survey were contacted by the researchers and agreed to sit for interviews, medical tests and even follow up mental ability tests.

"At one memorable meeting, around 100 people sat looking at the same test questions that they had sat for almost 70 years ago," said Deary. "I read out the same instructions as those teachers from 1932, and the same time limits were maintained."

"The results: first, that the 70-somethings scored quite a bit better than they did at age 11; second, that mental ability differences are pretty stable from age 11 to age 77; with some interesting exceptions, the high scorers did well and the modest remained so."

Other results arising from the comparison is that mental ability at age 11 is significantly associated with survival up to age 76 (the greater the ability, the better the chance of survival). On the other hand, against this trend, men with high mental ability were more likely to die in active service in World War II. People with lower mental ability were more prone to lung and stomach cancer. The research is ongoing and many more important findings are expected.